A photoelastic modulator (PEM) is an instrument that is used for modulating the polarization of a beam of light. A PEM employs the photoelastic effect as a principle of operation. The term “photoelastic effect” means that an optical element that is mechanically stressed and strained (deformed) exhibits birefringence that is proportional to the amount of deformation induced into the element. Birefringence means that the refractive index of the element is different for different components of a beam of polarized light.
A PEM includes an optical element, such as fused silica, that has attached to it one or more transducers for vibrating the optical element at a fixed frequency within, for example, the low-frequency, ultrasound range of about 20 kHz to 100 kHz. The mass of the element is compressed and extended as a result of the vibration. The combination of the optical element and the attached transducer(s) may be referred to as an optical assembly.
The compression and extension of the optical element imparts oscillating birefringence characteristics into the optical element. The frequency of this oscillating birefringence is determined by the size of the optical element and the speed of the transducer-generated vibration or sound wave through the material that comprises the optical element.
The effect of the oscillating birefringence of the PEM on a linear-polarized monochromatic light wave is to vary over time the phase difference between the orthogonal components of the light that propagates through the optical element. This phase difference is known as retardation or retardance and can be measured in terms of length, waves (for example, quarter-wave, half-wave), or phase angle. There are many scientific and commercial applications for which such modulated light is employed.
The optical assembly is contained within a housing or enclosure that includes an optical aperture through which the light under study is directed through the optical element. The enclosure supports the optical assembly in a manner that permits the optical element to be driven (vibrated) within the enclosure to achieve the above-noted photoelastic effect.
It is desirable to maximize the overall performance quality factor, or “Q” value, of the photoelastic modulator. In this regard, “Q” is defined as the ratio of the energy stored in a system to the energy lost per cycle. The higher the “Q,” the more efficient the system.
If an optical assembly is secured in the enclosure with somewhat rigid mounting mechanisms, the effect is to dampen the vibration of the optical element, thus requiring more drive energy to maintain the desired vibration frequency of the element. Increasing drive energy increases the heat generated within the photoelastic modulator, which causes a reduction in the Q value. Nonetheless, the optical assembly must be securely supported in a manner such that, apart from the vibration of the assembly, the optical assembly remains in a fixed position relative to the enclosure and optical aperture.
Moreover, the optical assembly should be supported in a way that permits vibration of the assembly without introducing any significant stress or strain on the optical element, which would affect the oscillating birefringence characteristics of the element.
The present invention provides an effective support for a vibrating member such as the optical assembly of a PEM. The support permits free vibration of the optical assembly with a high “Q” factor. Moreover, the support described here facilitates accurate and rapid assembly of the components of the optical assembly and enclosure.